Enna suffers from pseudobulbar affect (PBA), a neurological disorder brought on by nerve damage that makes it difficult to control one's emotional response. Some patients with PBA cry uncontrollably, others get angry, but for Enna, it has manifested as frequent bouts of the giggles.

'We were told by doctors that most patients who have a brain tumor removed feel depressed or angry afterwards. But when Enna came round she was giggling and it just carried on. She would giggle all the time – anything would set her off," Enna's mother Vana Stephens told the U.K. press.

While PBA can cause normal reactions, such as a chuckle following a joke, to become exaggerated, the emotional responses sometimes run contrary to the actual emotion the patient is feeling, or have nothing to do with it at all.

"Different patients suffer it in different ways," says Dr. Brian Greenwald, medical director of brain injury rehabilitation at the Mount Sinai Rehabilitation Center in New York City, who did not treat Enna. "Sometimes they'll be hysterically crying but not actually feel upset. Sometimes they are angry but it comes out as laughter. It can be incredibly frustrating to live with because it starts to interfere with one's social and professional life," he says.

In cases of traumatic brain injury, PBA can be a sign of damage to the brain and will often subside with time as the brain heals. For those with degenerative conditions, such as Parkinson's or Alzheimer's, however, PBA tends to worsen over time.

Though PBA is certainly not something one would wish for and Enna's PBA is expected to diminish, the little girl's parents note that this particular side effect has its silver lining:

"We'd visit Enna in hospital and try to put on a brave face but inside we were crushed. But once she started giggling we found ourselves doing the same. It was so infectious, and just a great way of releasing our emotions," Stephens told the U.K. press.

And according to Enna's doctors, there is something to smile about: they believe the cancer was completely removed and there is an 80 percent chance it won't return, doctors told the U.K. press.